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Ind. Eng. Chem. Res. 2006, 45, 2294-2302
Removal of Lignin and Associated Impurities from Xylo-oligosaccharides by Activated Carbon Adsorption Daniel Montane´ ,* De´ bora Nabarlatz, Anna Martorell, Vanessa Torne´ -Ferna´ ndez, and Vanessa Fierro Departament d’Enginyeria Quı´mica, ETSEQsRoVira i Virgili UniVersity, AV. Paı¨sos Catalans 26, 43007 Tarragona (Catalunya), Spain
This paper studies purification with commercial activated carbons of the xylo-oligosaccharides produced by the autohydrolysis of almond shells. Almond shells are agricultural residues with a high content of xylan that are produced abundantly in some regions with a Mediterranean climate. Adsorption equilibrium was measured in a batch system for three commercial activated carbons using a constant concentration of 20 g/L of crude xylo-oligosaccharides and loads of activated carbon from 1.5 to 50.0 mg/mL. Adsorption for lignin-related products was higher than for xylo-oligosaccharides and the selectivity toward lignin adsorption was better when the carbon was highly microporous and had small mesopore diameters, a low volume of mesopores, a low concentration of basic surface groups to limit xylo-oligosaccharide adsorption, and acidic surface groups to favor the adsorption of the lignin-related products. Column tests were performed at a feed rate of crude xylo-oligosaccharide solution of 6.0 mL/min (35 g/L) in columns packed with 22 g of granular activated carbon and operated in up-flow mode. Average retention was around 64% for lignin products and 21% for carbohydrates for the fraction of treated solution collected during the first 2 h of operation (13.1 bed volumes circulated through the bed). Retention for lignin-derived products was limited because part of them is linked to the xylo-oligosaccharides. Introduction Xylose-based oligosaccharides (xylo-oligosaccharides or xylooligomers) derived from xylan-rich hemicelluloses are carbohydrates with a high potential for novel applications in food and pharmaceutical products. As they are not metabolized by the human digestive system, xylo-oligosaccharides can be used as low-calorie sweeteners and soluble dietary fiber. They act as prebiotics, providing a source of carbon for the development of intestinal microflora and probiotic microorganisms,1-4 and are already used in fortified foods intended for the development of intestinal microflora.5,6 In addition, xylo-oligosaccharides have acceptable organoleptic properties and do not exhibit toxicity or negative effects on human health. Ethers and esters prepared from xylan and xylo-oligosaccharides have been synthesized and used as thermoplastic compounds for biodegradable plastics, water soluble films, coatings, capsules, and tablets7 and also for the preparation of chitosan-xylan hydrogels.8 Xylo-oligosaccharides extracted by autohydrolysis of bamboo have recently been found to possess a cytotoxic effect on human leukemia cells.9 The autohydrolysis of xylan-rich biomass is a suitable process for the production of xylo-oligosaccharides. It eliminates the use of important amounts of the chemicals needed in other extraction processes, such as alkali and acid, and since autohydrolysis takes place in slightly acidic media, many of the side chains in the backbone xylose chains, such as acetyl, uronic acids, and phenolic acid substituents, remain in the xylooligosaccharides.10,11 The differential characteristics of the substituted xylo-oligosaccharides obtained by autohydrolysis have prompted renewed interest in the development of process strategies for achieving a high yield of xylo-oligosaccharides * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (+34) 977 559 652. Fax: (+34) 977 558 544.
with consistent reproducibility in purity and composition. Though xylo-oligosaccharides are the main component in the nonvolatile products of biomass autohydrolysis, they are mixed with monosaccharides, ferulic acids, uronic acids, and compounds formed by the partial hydrolysis of lignin, the dehydration and degradation of carbohydrates, and condensation reactions. Lignin-derived products are the largest fraction of the impurities associated with xylo-oligosaccharides. Lignin is a three-dimensional polymer made of phenylpropane units linked randomly through alkyl-aryl ether bonds. It acts as a protecting agent and binder in the cell-wall structure of lignocellulosic materials. During autohydrolysis, lignin is depolymerized partially through cleavage of the ether linkages and yields phenolic monomers and oligomers that are soluble in the aqueous media. Also, some of the xylose units in the xylan backbone are bonded to lignin through ether and ester linkages. Consequently, some of the xylo-oligosaccharides contain lignin oligomers linked to the xylose chain. Various impurities from minor constituents of the lignocellulosic biomasssincluding inorganic salts, extractives, and, in some cases, proteinssare also present. Clearly, therefore, the crude xylo-oligosaccharides produced by the autohydrolysis of lignocellulosic biomass will contain large amounts of lignin-derived phenolics, carbohydrate dehydration and condensation products, and ash. For instance, the content of xylo-oligosaccharides in the nonvolatile products has been reported to be 58.3% for almond shells,12 54.8% for rice husks,13 and only 46.3% for barley residues.14 Crude xylooligosaccharides must be purified in order to obtain a final product that is well characterized chemically and structurally, homogeneous, repetitive, and suitable for food or pharmaceutical applications. Purification sequences based on liquid-liquid and solid-liquid extractions, solvent precipitation, and ion exchange treatments, as well as combinations of these techniques, have been thoroughly studied.13,14 Treatment with activated carbon has been shown to be an effective process for removing
10.1021/ie051051d CCC: $33.50 © 2006 American Chemical Society Published on Web 03/01/2006
Ind. Eng. Chem. Res., Vol. 45, No. 7, 2006 2295 Table 1. Surface Characteristics of the Commercial Activated Carbons (NORIT) Used in This Study Activated Carbons
SBET (m2/g) V0.99 (mL/g) VR,umic (
